Quantum information processing generally includes manipulation or use of quantum states to store or communicate information or to perform calculations. A variety of systems having quantum states have been proposed or used in quantum information processing. Optical systems, for example, can manipulate the quantum states of light to perform specific quantum information processing tasks.
A quantum computer architecture based on linear optical elements with nonlinearities induced by photodetection and feed-forward systems was originally proposed by E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature 409, 47 (2001). Although this proposal demonstrated that linear optics quantum computation (LOQC) was possible in principle, scalable systems based on this approach required an impractically large supply of quantum resources for reliable operation. Improvements to the proposal of Knill et al. have been developed (and experimentally demonstrated) requiring fewer resources, but these more recent proposals proscribe quantum circuit elements that behave probabilistically. For example, the quantum controlled-NOT gate described by T. B. Pittman, B. C. Jacobs, and J. D. Franson, “Probabilistic quantum logic operations using polarizing beam splitters,” Phys. Rev. A 64, 062311 (2001) requires fewer resources than corresponding systems proposed by Knill et al., but does not operate deterministically.
The system proposed by Pittman et al. uses measurement of one or more input photonic qubits and a first set of ancilla photonic qubits. The measurement results allow selection of one or more photonic qubits from a second set of ancilla photonic qubits that are entangled with the first set of ancilla photonic qubits. A problem with this technique is that the selected output photonic qubit has an inherent probability of being incorrect for the gate being implemented. The probability that the system will fail to produce the correct output is typically 75% (assuming perfect photodetectors). A linear quantum optical computer of this type having several such gates is thus extremely wasteful of offline quantum resources (e.g., entangled photons) and may be impractical for complex systems. For example, a quantum circuit including several linear optical quantum gates could perform a computation by operating those gates in parallel; the gates outputs can be teleported into the computation when the gates have functioned properly. Although this approach is scalable, it would require many repetitions of individual gate operations until the computation succeeded, thereby wasting many entangled and ancilla photons.
Optical quantum information processing systems are desired that are deterministic or otherwise efficiently utilize quantum resources. Ideally, such optical systems would also be suitable for miniaturization down to nanometer scales.